1. Field of the Invention
The present invention relates to an apparatus for detecting a defect in a sheet like material. More specifically, the present invention relates to an improvement in such a defect detecting apparatus adapted for detecting in a photoelectric manner a defect in a sheet like material while it is transferred during a weaving operation, such as through a sheet processing step, and the like.
2. Description of the Prior Art
In a weaving operation, for example, a defect such as a weaving flaw could occur in a weaved textile, because of a poor quality of a raw yarn, poor sizing, improper adjustment or maintenance of a weaving machine, and the like. Conventionally, therefore, it has been a common practice in the art that a defect in the weaved textile is detected during a weaving process or a testing process for the purpose of quality control of the weaved textile and for the purpose of enhancement of reliability to the product. Of late, various automatic or mechanical defect detecting apparatus and methods have been proposed and put in practical use. A typical automatic defect detection employs a photoelectric detection by the use of a light source and a photodetector provided relative to a textile sheet to be tested. One type of such photoelectric defect detecting system comprises a plurality of photodetectors arranged in the lateral direction of the textile sheet throughout the width of the textile sheet, wherein the photodetectors are fixedly provided, while the textile sheet is adapted to travel in the longitudinal direction. In view of the fixed provision of the photodetectors, this type may be referred to as "a fixed type". Another type of such photoelectric defect detecting system comprises a light source and a photodetector provided relative to a textile sheet, either or both of which is adapted to scan the textile sheet in the width direction. Hence, this type may be referred to as "a scanning type". The photoelectric defect detecting system of the so called scanning type can be classified into (1) an optical scanning type comprising a mirror rotatably provided to provide a scanning light beam in the width direction and (2) a mechanical scanning type comprising a detector provided to scan mechanically the textile sheet in the width direction. A fixed type defect detecting system is expensive in cost because a multiplicity of photodetectors are provided arranged in the lateral direction throughout the width of the textile sheet, which requires a complicated electrical circuit configuration. On the other hand, a defect detecting system of an optical scanning type requires a precise optical system, which also makes the system complicated and expensive in cost. By contrast, a defect detecting system of a mechanical scanning type can be simplified in structure and is more practical. Hence, it has been advantageously utilized as a defect detecting apparatus for a tricot knitting machine. Nevertheless, a mechanical scanning type defect detecting apparatus is disadvantageous in that it is of a slow scanning speed as compared with an optical scanning type apparatus. Hence, in the case of a sheet being tested of a wider width, it takes more time for a detector to again detect the same defect which was detected in the previous scanning cycle, with the result that it becomes impossible to determine whether the detected defect is a continuous one or separate ones.
As described previously, various types of defects could occur during a weaving process for various reasons. Particularly, a defect extending in the warp direction could be caused in the case where a warp defect preventing apparatus is not properly operated because the warp is snapped or loosened and is twisted with the peripheral portion of the warp sheet, and the like. A warp directional defect could be a relatively large continuous defect. Hence, it is desired that this type of warp directional continuous defect be detected as early as possible, thereby to avoid degradation of quality of the product in a weaving process.